The present embodiments refer to coding a sequence of digital images as well as to a corresponding decoding.
In many different applications (e.g., in surveillance systems or in medical imagery apparatus), a great amount of image and video data is produced. Hence, there is a need to compress this data in order to save storage capacity or to reduce the bandwidth when transmitting the data.
In the prior art, there exist a lot of standards in order to compress image and video data. Prominent examples of the standards are H.264/advanced video coding (AVC) see “Overview of the H.264/AVC Video Coding Standard,” by Thomas Wiegand, Gary J. Sullivan, Gisle Bjontegaard, and Ajay Luthra, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, Vol 13, No. 7, July 2003), as well as the draft standard high efficiency video coding (see “Overview of the High Efficiency Video Coding (HEVC) Standard,” Gary J. Sullivan, Jens-Rainer Ohm, Woo-Jin Han, and Thomas Wiegand, IEEE TRANS, ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, Vol. 22, No. 12, pp. 1649-1668, December 2012), which will be standardized also as ITU-T Recommendation H.265. The standard HEVC will also allow the real-time transmission of lossless coded image sequences. The standards HEVC and H.264/AVC include different intra prediction modes based on blocks in the same image. In those modes, a current block is predicted for already reconstructed pixels in the neighborhood. An encoder may test different prediction types and chooses the prediction type with minimal cost with respect to a certain distortion criterion. The prediction error is built for the current block and is transmitted to the decoder together with the prediction type. Block-wise prediction has the disadvantage that pixels that are far away from the reference pixels used for prediction do not correlate well with the reference pixels. Hence, the prediction error may be higher for those pixels. In order to improve the prediction, the size of a block may be reduced. However, this results in a higher number of blocks in an image, which leads to a higher bitrate for signaling of the prediction type. If the reference pixels contain noise, those pixels become suboptimal for prediction.
A simple and efficient pixel-wise prediction method is proposed in “The LOCO-I lossless image compression algorithm: Principles and standardization into JPEG-LS,” by Marcelo J Weinberger, Gadiel Seroussi, and Guillermo Sapiro, IEEE Transactions on Image Processing, August 2000. This prediction method named LOCO-I uses a simple and straight forward algorithm to predict a pixel based on three surrounding pixels. This prediction method is not optimal for compression for noisy images, either.
In “Intra prediction for lossless coding”, by Peter Amon, Andreas Hutter, Eugen Wige, and André Kaup, ISO/IEC JTCI/SC29/ WG11 and ITU-T SG16 WP3, document JCTVC-L0161/M27497, Geneva, Switzerland, January 2013, as well as in International patent application No. PCT/EP2012/075988, a template based prediction is disclosed where a predicted value of a pixel is determined by comparing a patch according to the template around the pixel with other patches according to the template around pixels in the neighborhood of the pixel to be predicted. As a result of this comparison, similarity measures between the patches are determined. The prediction is based on a weighted sum of the pixel values in the neighborhood taking into account a similarity measure, where lower similarity measures result in lower weights. This prediction method is rather complex and not suited for special types of contents (e.g., the content of a computer screen).